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{{Short description|Measure in population genetics}}
'''Genetic load''' is the difference between the [[fitness (biology)|fitness]] of an average [[genotype]] in a [[population]] and the fitness of some reference genotype, which may be either the best present in a [[population]], or may be the theoretically [[Maxima and minima|optimal]] genotype. The average individual taken from a population with a low genetic load will generally, when [[Transplant experiment|grown in the same conditions]], have more surviving offspring than the average individual from a population with a high genetic load.<ref name="Whitlock and Bourguet 2000">{{cite journal|last1=Whitlock|first1=Michael C.|last2=Bourguet|first2=Denis |year=2000 |title=Factors affecting the genetic load in ''Drosophila'': synergistic epistasis and correlations among fitness components |journal=Evolution |volume=54 |issue=5 |pages=1654–1660 |doi=10.1554/0014-3820(2000)054[1654:FATGLI]2.0.CO;2 |pmid=11108592|s2cid=44511613 |url=https://hal.archives-ouvertes.fr/hal-02914167/file/Whitlock%20and%20Bourguet%20Evolution%202000.pdf}}</ref><ref name="Crist and Farrar 1983">{{cite journal |last1=Crist |first1=Kathryn Carvey |last2=Farrar |first2=Donald R. |year=1983 |title=Genetic load and long-distance dispersal in ''Asplenium platyneuron'' |journal=Canadian Journal of Botany |volume=61 |issue=6 |pages=1809–1814 |doi=10.1139/b83-190}}</ref> Genetic load can also be seen as reduced fitness at the population level compared to what the population would have if all individuals had the reference high-fitness genotype.<ref>{{cite journal|author=JF Crow|author-link=James F. Crow|year=1958|title=Some possibilities for measuring selection intensities in man|journal=Human Biology |volume=30|pages=1–13|pmid=13513111|issue=1}}</ref> High genetic load may put a population in danger of [[extinction]].
==Fundamentals==
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===Deleterious mutation===
[[Deleterious mutation]] load is the main contributing factor to genetic load overall.<ref name="Klekowski 1988">{{cite journal|last1=Klekowski|first1=EdwardJ. |year=1988 |title=Genetic load and its causes in long-lived plants |journal=Trees |volume=2 |issue=4 |pages=195–203 |doi=10.1007/BF00202374|s2cid=24058154 }}</ref> The Haldane-Muller theorem of [[mutation–selection balance]] says that the load depends only on the deleterious [[mutation rate]] and not on the [[selection coefficient]].<ref>{{cite journal|last1=Bürger|first1=Reinhard|journal=Genetica|date=1998|volume=102/103|pages=279–298|doi=10.1023/a:1017043111100|title=Mathematical properties of mutation-selection models|s2cid=22885529}}</ref> Specifically, relative to an ideal genotype of fitness 1, the mean population fitness is <math>\exp(-U)</math> where U is the total deleterious mutation rate summed over many independent sites. The intuition for the lack of dependence on the selection coefficient is that while a mutation with stronger effects does more harm per generation, its harm is felt for fewer generations.
A slightly deleterious mutation may not stay in mutation–selection balance but may instead become [[fixation (population genetics)|fixed]] by [[genetic drift]] when its [[selection coefficient]] is less than one divided by the [[effective population size]].<ref>{{cite journal|last1=Lande|first1=Russell|title=Risk of Population Extinction from Fixation of New Deleterious Mutations|journal=Evolution|date=October 1994|volume=48|issue=5|pages=1460–1469|doi=10.2307/2410240|pmid=28568413|jstor=2410240}}</ref> In asexual populations, the [[stochastic]] accumulation of mutation load is called [[Muller's ratchet]], and occurs in the absence of beneficial mutations, when after the most-fit genotype has been lost, it cannot be regained by [[genetic recombination]]. Deterministic accumulation of mutation load occurs in asexuals when the deleterious mutation rate exceeds one per replication.<ref name="kondrashov">{{cite journal | doi = 10.1038/336435a0 | last1 = Kondrashov | first1 = A. S. | author-link = Alexey Kondrashov | year = 1988| title = Deleterious mutations and the evolution of sexual reproduction | journal = [[Nature (journal)|Nature]] | volume = 336 | issue = 6198| pages = 435–440 | pmid=3057385| bibcode = 1988Natur.336..435K| s2cid = 4233528 }}</ref> Sexually reproducing species are expected to have lower genetic loads.<ref>{{cite thesis |author=Marriage, Tara N. |year=2009 |title=Mutation, asexual reproduction and genetic load: A study in three parts |publisher=University of Kansas |type=Ph.D. thesis |url=https://kuscholarworks.ku.edu/handle/1808/5949}}</ref> This is one hypothesis for the [[evolution of sexual reproduction|evolutionary advantage of sexual reproduction]]. Purging of deleterious mutations in sexual populations is facilitated by [[epistasis|synergistic epistasis]] among deleterious mutations.<ref name="crow 97">{{cite journal|last1=Crow|first1=James F.|title=The high spontaneous mutation rate: Is it a health risk?|journal=Proceedings of the National Academy of Sciences|date=5 August 1997|volume=94|issue=16|pages=8380–8386|language=en|issn=0027-8424|doi=10.1073/pnas.94.16.8380|pmid=9237985|pmc=33757|bibcode=1997PNAS...94.8380C|doi-access=free}}</ref>
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High load can lead to a [[small population size]], which in turn increases the accumulation of mutation load, culminating in [[extinction]] via [[mutational meltdown]].<ref>{{cite journal|last1=Lynch|first1=Michael|last2=Conery|first2=John|last3=Burger|first3=Reinhard|title=Mutational Meltdowns in Sexual Populations|journal=Evolution|date=December 1995|volume=49|issue=6|pages=1067–1080|doi=10.2307/2410432|pmid=28568521|jstor=2410432}}</ref><ref>{{cite journal|last1=Lynch|first1=Michael|last2=Conery|first2=John|last3=Burger|first3=Reinhard|title=Mutation Accumulation and the Extinction of Small Populations|journal=The American Naturalist|date=1 January 1995|volume=146|issue=4|pages=489–518|jstor=2462976|doi=10.1086/285812|s2cid=14762497}}</ref>
The accumulation of deleterious mutations in humans has been of concern to many geneticists, including [[Hermann Joseph Muller]],<ref>{{cite journal|last1=Muller|first1=H. J.|title=Our load of mutations|journal=American Journal of Human Genetics|date=1 June 1950|volume=2|issue=2|pages=111–176|pmc=1716299|issn=0002-9297|pmid=14771033}}</ref> [[James F. Crow]],<ref name="crow 97" /> [[Alexey Kondrashov]],<ref>{{cite journal|last1=Kondrashov|first1=Alexey S.|title=Contamination of the genome by very slightly deleterious mutations: why have we not died 100 times over?|journal=Journal of Theoretical Biology|date=21 August 1995|volume=175|issue=4|pages=583–594|doi=10.1006/jtbi.1995.0167|pmid=7475094|bibcode=1995JThBi.175..583K |doi-access=free}}</ref> [[W. D. Hamilton]],<ref>{{cite book|last1=Hamilton|first1=W.D.|title=Narrow Roads of Gene Land vol. 2: Evolution of Sex|pages=449–463}}</ref> and [[Michael Lynch (geneticist)|Michael Lynch]].<ref>{{cite journal|last1=Lynch|first1=M.|title=Mutation and Human Exceptionalism: Our Future Genetic Load|journal=Genetics|date=7 March 2016|volume=202|issue=3|pages=869–875|doi=10.1534/genetics.115.180471|pmid=26953265|pmc=4788123}}</ref>
====Beneficial mutation====
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Inbreeding increases [[Zygosity#Homozygous|homozygosity]]. In the short run, an increase in inbreeding increases the probability with which offspring get two copies of a recessive deleterious alleles, lowering fitnesses via [[inbreeding depression]].<ref name="Saccheri et al. 2005">{{cite journal|last1=Saccheri|first1=I. J.|last2=Lloyd|first2=H. D.|last3=Helyar|first3=S. J.|last4=Brakefield|first4=P. M. |year=2005 |title=Inbreeding uncovers fundamental differences in the genetic load affecting male and female fertility in a butterfly|journal=Proceedings of the Royal Society B: Biological Sciences |volume=272 |issue=1558 |pages=39–46 |doi=10.1098/rspb.2004.2903 |pmid=15875568 |pmc=1634945}}</ref> In a species that habitually inbreeds, e.g. through [[selfing|self-fertilization]], a proportion of recessive deleterious alleles can be [[genetic purging|purged]].<ref name="Byers and Waller 1999">{{cite journal|last1=Byers|first1=D. L.|last2=Waller|first2=D. M. |year=1999 |title=Do plant populations purge their genetic load? Effects of population size and mating history on inbreeding depression |journal=Annual Review of Ecology and Systematics |volume=30 |issue=1 |pages=479–513 |doi=10.1146/annurev.ecolsys.30.1.479}}</ref><ref name="Barrett and Charlesworth 1991">{{cite journal|last1=Barrett |first1=S. C. H. |last2=Charlesworth |first2=D. |year=1991 |title=Effects of a change in the level of inbreeding on the genetic load |journal=Nature |volume=352 |issue=6335 |pages=522–524 |doi=10.1038/352522a0 |pmid=1865906|bibcode=1991Natur.352..522B |s2cid=4240051 }}</ref>
Likewise, in a small population of humans practicing [[endogamy]], deleterious alleles can either overwhelm the population's gene pool, causing it to become extinct, or alternately, make it fitter.<ref name="Pala and Zappala 2017">{{cite journal|last1=Pala |first1=M. |last2=Zappala |first2=Z. |last3=Marongiu|first3=M.|year=2017 |title=Population and individual-specific regulatory variation in Sardinia |journal=Nature Genetics |volume=49| pages=700–707 |issue= 5|doi=10.1038/ng.3840|pmid= 28394350|pmc=5411016 |bibcode= |s2cid=4240051
===Recombination/segregation===
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===Migration===
Migration load is hypothesized to occur when maladapted non-native organisms enter a new environment.<ref>{{cite journal|last1=Bolnick|first1=Daniel I. |year=2007 |title=Natural selection in populations subject to a migration load |journal=Evolution |volume=61 |issue=9 |pages=2229–2243 |doi=10.1111/j.1558-5646.2007.00179.x|pmid=17767592|s2cid=25685919|doi-access= }}</ref>
On one hand, beneficial genes from migrants can increase the fitness of local populations.<ref name="Hu">{{cite journal|last1=Hu|first1=Xin-Sheng|last2=Li|first2=Bailian |year=2003 |title=On migration load of seeds and pollen grains in a local population |journal=Heredity |volume=90 |issue=2 |pages=162–168 |doi=10.1038/sj.hdy.6800212 |pmid=12634823|doi-access=free }} "Gene flow can homogenize the genetic divergence among populations. On the one hand, effects of genetic drift in small local populations can be effectively reduced when the average number of migrants is greater than one (Wright, 1969), beneficial immigrant genes can shift local populations to a higher fitness peak (Barton and Whitlock, 1997). On the other hand, gene flow between populations adapted to different environments can cause maladaptation in a recipient population, resulting in migration load, a reduction in population fitness. If the migration rate is much greater than the selection coefficient, migrant alleles can even swamp out locally adaptive alleles (Wright, 1969)."</ref> On the other hand, migration may reduce the fitness of local populations by introducing maladptive alleles. This is hypothesized to occur when the migration rate is "much greater" than the selection coefficient.<ref name="Hu" />
Migration load may occur by reducing the fitness of local organisms, or through natural selection imposed on the newcomers, such as by being eliminated by local predators.<ref>{{harvnb|Bolnick|2007|ps=: "A second consequence of migration–selection balance is known as “migration load” (Garcia-Ramos and Kirkpatrick 1997). This is the loss in mean fitness of a population that results from immigration of locally maladapted alleles. Migration load is analogous to the “mutation load” that arises when mutation inputs new alleles that, on average, are expected to be less fit than existing alleles. Although both migration and mutation have the potential to import locally beneficial novel alleles that promote adaptation (Kawecki 2000), immigrants from other environments may frequently carry alleles that are less fit in the local habitat. Consequently, in addition to constraining divergence among populations, migration displaces recipient populations from their local adaptive peaks. For Mendelian traits, this entails a reduction in the frequency of locally favored alleles that otherwise would be at or near fixation, whereas quantitative traits may be displaced from the mean value favored by selection."}}</ref><ref>{{harvnb|Bolnick|2007|ps=:"Given this life history, the homogenizing effects of migration are expected to be most obvious early in a generation, after which natural selection by visual predators presumably eliminates many immigrants."}}</ref> Most studies have only found evidence for this theory in the form of selection against immigrant populations, however, one study found evidence for increased mutational burden in recipient populations, as well.<ref>{{harvnb|Bolnick|2007|ps=: To date, support for migration load has generally been confined to studies of individual populations, in which selection operates against immigrants (King 1992; Sandoval 1994a; Hendry et al. 2002; Moore and Hendry 2005).}}</ref>
==References==
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